American B738 at Montreal on Nov 30th 2010, runway excursion
Last Update: November 5, 2013 / 18:05:25 GMT/Zulu time
Incident Facts
Date of incident
Nov 30, 2010
Classification
Incident
Cause
Runway excursion
Airline
American Airlines
Aircraft Type
Boeing 737-800
ICAO Type Designator
B738
Findings as to Causes and Contributing Factors
- Following a stabilized approach and normal landing, the aircraft deviated left of the runway centerline, likely as the result of a nose-gear steering metering low-slew rate jam.
- The delayed response to the uncommanded steering event by the pilot flying was not sufficient to counteract the movement toward the left, and the aircraft departed the runway surface.
Findings as to Risk
- In the absence of information on uncommanded steering events due to nose-gear steering rate jams, there is a risk that the cause of these events will continue to be unresolved and unmitigated, leading to a risk of runway excursions.
- The lack of flight data recorder information or other types of recording devices on the nose-gear steering system may hinder the identification of safety deficiencies.
Other Findings
- The flight operational quality assurance programs in place at many airlines already target certain events with a view to underlining safety concerns. With additional filters, it would be possible to flag steering events in order to help in verifying for rate-jam events.
The TSB reported the captain (ATPL, about 15,000 hours total, about 200 hours on type) was pilot flying, the first officer (about 10,000 hours total, about 6,800 hours on type) was pilot monitoring.
The flight from Dallas Ft. Worth to Montreal was uneventful. The crew prepared for a landing on runway 24R, which was reported bare and wet in light rain, Vref was computed at 145 KIAS with flaps at 30 degrees, autobrakes probably were set to 2 (which is consistent with the actual deceleration rate), the TSB annotated the settings were appropriate for the runway conditions and length.
While overflying the final approach fix the aircraft encountered winds from 180 degrees magnetic at 44 knots, which gradually decreased to 157 degrees magnetic and 14 knots over the runway threshold. The approach was stable with only minor deviations from localizer and glideslope, the autopilot was disengaged at 1830 MSL with autothrottle remaining engaged until touchdown. The aircraft crossed the runway threshold at 32 feet AGL and 156 KIAS (Vref+11) and touched down firmly 825 feet past the threshold at 150 KIAS(Vref+5), the speedbrakes deployed automatically and autobrakes engaged within 1 second after touchdown. Reverse thrust was selected manually simultaneously, the aircraft slowed normally.
The aircraft tracked the runway center line for about 16 seconds with minor deviations only, but then began to veer left about 4370 feet down the runway. 3 seconds after the aircraft began to veer left the captain applied right control wheel input, another 4 seconds later right pedal input. 9 seconds after the aircraft began to veer left the right control wheel input reached 90 degrees, right rudder was deflected 83% of its possible travel, the aircraft finally responded and began to turn right towards the runway center line again, however continued to drift towards the left hand edge of the runway, the flight data recorder suggested the aircraft had a slight sideways travel even though there was no evidence the tyres or wheels had locked up. The aircraft was abeam taxiway E about 60 feet left of the runway center line when the pilot flying released the right hand pedal input and increased thrust to maximum reverse reaching the MAX reverse detent.
13 seconds after the aircraft began to veer left the aircraft exited the runway to the left 61 feet past taxiway E at a heading of 230 degrees magnetic and a speed of 59 knots over ground and came to a stop 22 seconds after beginning to veer left with all gear on soft ground about 90 feet off the left runway edge and 6600 feet down the runway.
Automatic braking was active all times, no manual braking was attempted. There had been no indications of any anomaly. The crew radioed tower that they had departed the runway, worked the relevant checklists and decided in the absence of any hazardeous scenario that an evacuation was not required. The passengers therefore disembarked via mobile stairs onto soft ground.
The aircraft sustained minor damage to the engines due to some light foreign object ingestion, that damage was considered within tolerances however. The tyres received a few chevron marks, but showed no other damage, especially no reverted rubber damage. The left main landing-gear lower torque-link bracket was found bent.
One runway light was damaged when the aircraft departed the runway. There were no tyre marks or indications of aquaplaning on the runway surface.
Discussions with Boeing, the NTSB and the airline identified a number of possible scenarios that might be able to explain the runway excursion, simulator tests however ruled all those options out except a nose wheel steering anomaly. The TSB stated: "Boeing believes that the most likely cause of the uncommanded steering input was a temporary, low slew-rate, nose-gear steering rate jam." and explained: "A nose-gear steering rate jam occurs when the feedback to the steering system is interrupted, causing the nose gear to continue turning (slewing) at a fixed rate until full travel is reached or until the rate jam is eliminated."
The TSB analyzed: "The crew performed a stable approach and normal landing at night, in light rain conditions. Runway conditions were reported to be bare and wet. Braking conditions were considered to be good. All aircraft systems were operating normally on approach and landing during the initial deceleration and landing roll. A detailed examination of the flight data recorder (FDR) data indicated that the left veer from runway centerline, 16 seconds after landing, was uncommanded by the crew."
The TSB applauded the crew for pulling the CVR circuit breaker stating "It is often the case that this item is forgotten".
The TSB further analyzed: "Besides the fact that the crew members were unaware that an uncommanded steering event could occur from a nose-gear steering rate jam, there were no warning horns, lights, alerts, noise, or vibration to indicate to the crew that a steering problem existed. The only indication was that the aircraft was slowly veering left. Several factors may have led to confusion and delayed the PF’s response, such as the following:
- The landing was executed with a 16-knot left-hand crosswind, but was within limits.
- The aircraft was drifting toward the left side of the runway into wind.
- The runway was wet but not flooded.
- The aircraft felt like it was sliding sideways on ice.
- Outside air temperature was above freezing, which normally would prevent the possibility of ice on the runway; there had been no need to prepare for landing on a slippery runway.
- Once normal right rudder pedal input was applied, the aircraft did not respond immediately; more than the usual amount of rudder pedal was necessary.
- Visual cues would have been less evident, due to the darkness and light rain on the windscreen.
- There were no vibrations, noises, or other indications of a steering abnormality."
The TSB further analyzed with regards to the cause of the steering anomaly: "As in previous valve teardowns following uncommanded steering events, the teardown of this nose-gear steering metering valve from the occurrence aircraft did not reveal any anomalies or operational difficulties that could explain its role in the uncommanded steering events. Nosewheel steering was tested and found to be functional after this occurrence, and no further directional control difficulties were reported between the time of the occurrence in November 2010 and the time the nose-gear steering metering valve was removed for examination in March 2011."
Boeing assessed the probability of such an event as 1 in 10 million cycles, in a period of 21 years 11 such occurrences had been attributed to a nose gear steering rate jam rendering the probability as "extremely unlikely". Therefore the TSB analyzed: "Boeing has not conducted any further testing or research to confirm the second possible cause of a rate jam, which is the interruption of feedback somewhere in the external linkage input to the metering valve. Aircraft behaviour would show the same low-slew rate jam as with a jam within the valve. The nose-gear wheel well, which includes the nose-gear steering assembly, is exposed to the outside elements, such as dirt, sand, stones, ice, snow, and water. Despite the presence of a plastic cover positioned on the nose-gear assembly, debris could possibly become jammed and affect the assembly’s normal operation."
Due to the low probability of such an event Boeing had not informed operators nor were they required to do do about the possibility of such an event. Crews therefore remained unaware of that possibility.
Boeing maintained that even in case of such a steering rate jam rudder authority would be sufficient to keep the aircraft on the runway provided corrections are applied sufficiently quick.
Incident Facts
Date of incident
Nov 30, 2010
Classification
Incident
Cause
Runway excursion
Airline
American Airlines
Aircraft Type
Boeing 737-800
ICAO Type Designator
B738
This article is published under license from Avherald.com. © of text by Avherald.com.
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